The navigation messages sent by the global satellite navigation systems (such as Galileo, GPS, etc.) with their added systems which increase their integrity (EGNOS, WAAS, etc.) also include orbit prediction information for these satellites and reference time information. Using these predictions, the users of these navigation services compute their estimated position based on pseudo-distance measurements.
Any error affecting these predictions results in an error on the value of the geographic position of the user determined in this way. In the case of services for which safety is a vital issue, an integrity monitoring segment is provided for monitoring the integrity of the users by checking the validity of the predictions, and broadcasting the integrity information (integrity indicators such as SISA/SISMA for Galileo or UDRE for EGNOS/WAAS, which is a European superlayer for the GPS system). The users take account of these indicators in order to estimate the risk that the error affecting the position indicated is greater than an alarm threshold. The standardization and the certification of these indicators and of the algorithms for implementing them are currently under way.
One of the main problems raised by the community of satellite navigation experts is that, with the current standards, the integrity of the measurements can be obtained only on the imperative mathematical condition that the distribution of the errors is not affected by systematic effects, in particular bias. If such is not the case, even if the errors of each satellite are individually controlled via an allowance level broadcast to the users, the resulting error at the user's level cannot be controlled.
Given that the bandwidth of the navigation data broadcast channels is strictly limited, it is impossible to broadcast more than one orbit and clock prediction error characterization parameter.
To resolve this problem, attempts have been made to artificially increase the integrity indicators so as to improve the allowance level of the distribution of the prediction errors. However, this artificial increase has a strong influence on the availability of the integrity service, because it leads to numerous false alarms. It was therefore necessary to improve the quality of the corrections of the orbit and clock data.
The limitations inherent in the quality of the predictions are not always clearly identified. Numerous factors may be the cause of this limitation. Such factors include, in particular:                the inaccuracy of the model of the Earth's gravitational field,        the fact of neglecting the higher-order tidal effects or the multiple-body effects (for example, the other planets of the solar system),        algorithms for extrapolating observations into predictions that are insufficiently tried and tested,        instability of the onboard navigation signal generation hardware,        inaccuracy of the modeling of the solar radiation pressure effect,        inaccuracy of the satellite mass data, of the position of the satellite's center of gravity, etc.,        and other factors not currently identified.        
Most of the factors cited above are either inherent to the limitations of the state of the art (the first four), or subject to parameter drifts throughout the life of the satellite (the last three). The current solutions that are implemented to reduce the impact of these factors are primarily the following:                improvement of the accuracy of the geophysical data, for example improvement of the quality of the models of the Earth's gravitational field or of the tides,        improvement of the accuracy of the data relating to the current state of the satellite,        improvement of the performance levels of the computation circuits in order to enable them to compute a larger quantity of data with greater accuracy for longer iterative processes,        improvement of the stability of the onboard hardware (by thermal control, refinement of the electronic circuits, etc.).        
All these known solutions present limitations, in particular:                the accuracy of the geophysical data can be improved only at a relatively slow pace, from analysis of the results of the scientific missions,        the accuracy of the data relating to the equipment and to the components of the satellite is very limited because of the very limited possibilities for observing them after the satellite has been launched. For example, such is the case with parameters such as the optical reflectivity of the satellite following the inevitable degradation of its reflective outer coating which plays a predominant role in the solar radiation pressure effect,        it is not possible to improve the computation performance levels more rapidly than it is possible to improve the performance levels of the integrated circuits, which may soon reach their maximum level when their miniaturization reaches the atomic scale.        